The use of sulfide-type antioxidants as stabilizers in polyolefin resin compositions is known in the art.
For example, the use of diesters of thiodipropionic acid, such as the dilauryl, dimyristyl and distearyl esters of the thiodipropionic acid, as well as thiodipropionic acid itself, as antioxidants in crosslinked polyolefins is known in the art. For example, U.S. Pat. Nos. 3,876,613 and 4,028,332 disclose the use of such diesters of thiodipropionic acid in producing useful molded articles from peroxide-induced crosslinked polyethylene homopolymers and copolymers. The resultant crosslinked copolymers have high impact strength and a high resistance to stress cracking.
Likewise, U.S. Pat. No. 4,514,539 teaches the use of thiodipropionic esters as antioxidants in resin systems which comprise a combination of ethylene-vinyl acetate copolymers and polyethylene. The resin systems are crosslinked with peroxides. U.S. Pat. No. 4,526,916 discloses the use of thiodipropionate esters as antioxidants in peroxide-induced crosslinked polyethylene compositions, which are capable of being used in rotational molding processes.
U.S. Pat. No. 4,221,699 teaches the use of thio-bisphenols as antioxidants in polyolefin compositions which are also crosslinked with peroxides.
The crosslinking of polymer compositions has the effect of changing a plastic from a thermoplastic material to thermosetting material. This change has the effect of increasing the strength, heat and electrical resistance of the plastic, in addition to increasing its resistance to solvents, chemicals, creep and stress cracking. (See, e.g., The Condensed Chemical Dictionary, 10th Ed., p. 287 (1981)). Furthermore, the crosslinking of polymer compositions extends the useful upper temperature limit of the polymer as well as improves the heat-shrinking properties of the polymer, discussed in The Encyclopedia of Polymer Science and Engineering, 2nd Ed., Vol. 4, p. 385 (1986). The types of polymers which can be crosslinked in order to cause an increase in their mechanical properties are known to those skilled in the art and are discussed in numerous references, such as, e.g., The Encyclopedia of Polymer Science and Engineering, 2nd Ed. Vol. 4, pp. 418-449 (1986).
Crosslinked polyolefin resin compositions are useful in a wide variety of applications. For example, crosslinked polyolefin compositions are useful as wire and cable coatings, insulation, pipe and molded fittings, and rotational molded articles, such as gasoline tanks, barrels, containers, storage tanks, etc.
As indicated above, the use of organic sulfides as antioxidants in polyolefin resin compositions is known in the art. Particularly, the sulfide antioxidants are conventionally used in combination with various phenolic and arylamine auxiliary antioxidants to stabilize polyolefin resin compositions against the damaging effects of thermal and oxidative degradation both during processing and aging. For example, the use of organic sulfides as stabilizers in polyolefin resin compositions is taught in U.S. Pat. Nos. 3,180,850, 3,258,493, 3,293,209, 3,574,165, 3,652,680 and 3,772,246. However, conventional antioxidants have demonstrated certain drawbacks when used in polyolefin resins which are crosslinked.
Polyolefin resin compositions may generally be crosslinked through the use of (1) peroxide crosslinking agents or (2) high energy radiation. For polyolefin resin compositions crosslinked with peroxide, the range of useful antioxidants is limited, since the antioxidant itself must be compatible with the peroxide crosslinking agent, as discussed in U.S. Pat. No. 4,028,332. As further discussed in this patent, the thiodipropionate ester antioxidants, such as dilauryl thiodipropionate (DLTDP), are peroxide-compatible and provide a good low temperature impact strength, at least in polyolefin resins. However, these thiodipropionate ester antioxidants suffer several drawbacks. Namely, the antioxidant causes the resultant polymer compositions to discolor and to display an unpleasant odor.
Likewise, when high energy radiation is employed to induce crosslinking in the polyolefin resin compositions, the same criteria set forth above for the peroxide-induced crosslinked polymer compositions holds true. Similarly, conventional antioxidants used with polymer composition crosslinked with high energy radiation may cause the polymer compositions to discolor and have an unpleasant odor. The antioxidants used in these types of crosslinked polyolefin resins must be stable in the presence of the high energy radiation while providing its stabilizing effects.
Accordingly, one of the objects of the present invention is to provide crosslinked polymer compositions stabilized with organic sulfide antioxidants which are stable to the effects of heat and oxygen, and provide stabilized resin compositions which are hydrolytically stable, odor-free and color stable.